Self-sealing membrane for MEMS devices

ABSTRACT

Embodiments of the present disclosure are related to MEMS devices having a suspended membrane that are secured to and spaced apart from a substrate with a sealed cavity therebetween. The membrane includes openings with sidewalls that are closed by a dielectric material. In various embodiments, the cavity between the membrane and the substrate is formed by removing a sacrificial layer through the openings. In one or more embodiments, the openings in the membrane are closed by depositing the dielectric material on the sidewalls of the openings and the upper surface of the membrane.

BACKGROUND

1. Technical Field

Embodiments of the present disclosure relate to micro-electromechanical(MEMS) devices, and in particular, MEMS devices having suspendedmembranes.

2. Description of the Related Art

Membrane MEMS devices, such as pressure sensors and microphones, areused in various applications. Typically, membrane MEMS devices, such asthe device 10 shown in FIG. 1, includes a structural layer 12 having aportion that is supported by a substrate 14 and a portion forming asuspended membrane 13 that is separated from the substrate 14 by acavity 16.

In some cases, membrane MEMS devices are fabricated using surfacemicromachining to release or suspend the membrane. Generally described,surface micromachining includes forming a sacrificial layer on a surfaceof a substrate. A structural layer is then formed on the sacrificiallayer. Openings are formed in the structural layer, and the sacrificiallayer is removed through the openings thereby releasing the membrane andforming a cavity.

For various applications, the cavity is sealed from atmosphere and setat a particular pressure. In that regard, the openings used to removethe sacrificial layer will be closed, sealing the cavity.

In the past, the openings have been closed by an indirect sealing, suchas is shown in the device 10 of FIG. 1. In particular, a cover 18 isprovided over the structural layer 12 so that the space within the cover18, including the cavity 16, is sealed from atmosphere. The cover 18 maybe secured to the substrate 14, such as by anodic bonding.

BRIEF SUMMARY

Embodiments of the present disclosure are related to MEMS devices havinga suspended membrane that are secured to and spaced apart from asubstrate with a sealed cavity therebetween. The membrane includesopenings with sidewalls that are closed by a dielectric material. Invarious embodiments, the cavity between the membrane and the substrateis formed by removing a sacrificial layer through the openings. In oneor more embodiments, the openings in the membrane are closed bydepositing the dielectric material on the sidewalls of the openings andthe upper surface of the membrane.

In one embodiment, there is provided a method of forming a membrane MEMSdevice. The method includes forming a sacrificial layer over a surfaceof a substrate and forming a structural layer over the sacrificiallayer. The method further includes forming a plurality of openings withsidewalls in the structural layer. The method further includes releasinga portion of the structural layer by removing the sacrificial layerthrough the plurality of openings in the structural layer. The methodfurther includes closing the openings by depositing a dielectricmaterial on the sidewalls of the openings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a partial schematic side view of a membrane MEMS device.

FIG. 2A is a partial schematic side view of a membrane MEMS deviceaccording to an embodiment of the present disclosure.

FIG. 2B is a partial plan view of the membrane MEMS device of FIG. 2A.

FIGS. 3A-3F are partial schematic side views of various stages forforming the membrane MEMS device of FIG. 2.

FIGS. 4A and 4B are partial schematic side views of the effects ofarrival angle during PECVD deposition.

DETAILED DESCRIPTION

FIG. 2A is a partial schematic side view of a membrane MEMS device 20that has been formed according to an embodiment of the presentdisclosure. Generally described, the membrane MEMS device 20 includes astructural layer 12 that includes a suspended membrane 13 that is spacedapart from an upper surface 22 of a substrate 14 by a cavity 16. A legportion 24 of the structural layer 12 located outwardly of the suspendedmembrane 13 is secured to the substrate 14. In one embodiment, thesubstrate 14 is a semiconductor material, such as monocrystallinesilicon.

The substrate 14 may have one or more layers formed in and/or on theupper surface 22. For instance, in one embodiment the membrane MEMSdevice 20 is a capacitive pressure sensor. In that regard, the substrate14 may have a conductive layer formed on the upper surface 22 of thesubstrate 14 in the cavity 16 such that the conductive layer is spacedapart from the suspended membrane 13. The conductive layer on thesubstrate 14 is a fixed electrode and the suspended membrane 13 is themoveable electrode of the capacitor. In other embodiments, the substrateitself is formed to act as the fixed electrode for the capacitor. Inanother embodiment, the substrate 14 has logic circuits and transistorsformed therein. They might be below the upper surface 22 or off to oneside.

Depending on the application, the cavity 16 of the device 20 is set at aparticular pressure and sealed from atmosphere. In the embodiment inwhich the device 20 is a pressure sensor, the membrane 13 is made fromany suitable material and has a suitable thickness that allows themembrane 13 to move in response to pressure being applied to its uppersurface 26. In some embodiments, the membrane material is electricallyconductive, such as a metal material. In another embodiment, themembrane material is a semiconductor material, such as single crystalsilicon or polysilicon. In yet another embodiment, the membrane 13 ismade of a plurality of layers of various materials. In one embodiment,the membrane is aluminum and has a thickness of less than 1 micron.

The membrane 13 includes openings 30 having sidewalls 32 that extendfrom the upper surface 26 of the membrane 13 to a lower surface 34 ofthe membrane 13. A dielectric material 40 is located on the uppersurface 26 of the membrane 13 and along the sidewalls 32 of the openings30 to at least partially fill the openings 30 so as to seal the cavity16. The dielectric material 40 may be any material configured to adhereto the sidewalls 32 of the openings 30. In one embodiment, thedielectric material 40 is silicon nitride.

In the illustrated embodiment, some of the dielectric material 40 isalso located on the upper surface 22 of the substrate 14 as will beexplained in more detail below. It is to be appreciated that dielectricmaterial 40 on the upper surface 22 of the substrate 14 does notsubstantially affect the operation of the membrane MEMS device 20. Inother embodiments, the membrane MEMS device 20 may also includedielectric material 40 on the portion 24 of the structural layer 12 oron the entire outer surface of the structural layer 12.

In FIG. 2B, which is a plan view of the membrane 13 of the membrane MEMSdevice 20 of FIG. 2A, the openings 30 are shown in dotted line toindicate that they are located beneath the dielectric material 40. Theopenings 30 are formed in the portion of the structural layer 12 thatforms the suspended membrane 13 and are generally not formed in theportion 24 of the structural layer 12 that is used to secure thestructural layer 12 to the substrate 14. Although the illustratedembodiment shows an array of openings 30, the openings 30 may bearranged in a different pattern.

As will be explained below, the openings 30 are used to remove asacrificial layer and suspend the membrane 13. Although the openings areillustrated in a circular shape, the openings 30 may be of any shapesuitable to suitably allow removal of the sacrificial layer 42.

FIGS. 3A-3F are partial schematic side views of various stages forforming the membrane MEMS device 20 according to one embodiment. Asshown in FIG. 3A, a sacrificial layer 42 is formed over the uppersurface 22 of the substrate 14 as is well known in the art. Thesacrificial layer 42 is an insulating material, such as oxide,polyimide, or a combination thereof. The structural layer 12 is formedover the sacrificial layer 42 and is secured to the substrate 14 atportion 24.

As shown in FIG. 3B, openings 30 are formed in the portion of thestructural layer 12 that forms the suspended membrane 13. As discussedabove, the openings 30 have sidewalls 32 that extend through the entirethickness of the structural layer 12 thereby exposing the sacrificiallayer. As is well known in the art, the openings 30 may be formed byforming a patterned mask layer over the structural layer 12 and etchingthe structural layer 12 to form the openings 30. In another embodiment,the structural layer 12 may be formed over the sacrificial layer 42 withthe openings 30.

As shown in FIG. 3C, the sacrificial layer is removed through theopenings 30 in the structural layer 12, suspending the membrane 13 andforming the cavity 16 between the suspended membrane 13 and thesubstrate 14. In particular, the openings 30 may be removed by etchingthe sacrificial layer 42 through the openings 30. As is well known inthe art, the sacrificial layer 42 may be removed by a wet etch process,such as by hydrofluoric acid.

In another embodiment, the sacrificial layer 42 may be removed by a dryetch process. In such an embodiment, the sacrificial layer 42 may be,for example, polyimide. By using a dry etch process to remove thesacrificial layer 42, various benefits may be obtained. One such benefitis that the openings 30 formed in the membrane 13 may have smallerdimensions than when a wet etch process is used. That is, in a wet etchprocess the dimensions of the openings need to be sufficiently large toallow the fluid to flow there-through. By forming smaller openings, theopenings will be easier to close as will be discussed in more detailbelow. Furthermore, using a dry etch process rather than a wet etchprocess also resolves problems associated with agents from the wet etchprocess remaining trapped in the cavity. These agents can cause a riskof impairing the functionality of the membrane. For instance, capillaryaction caused by the trapped liquid can be sufficient to force themembrane 13 toward the substrate 14, and in some cases cause themembrane 13 to collapse.

After the sacrificial layer 42 has been suitably removed to suspend themembrane 13, the openings 30 are closed to seal the cavity 16. Inparticular, the openings 30 may be closed by depositing the dielectricmaterial 40 along the sidewalls 32 of the openings 30 and on the uppersurface 26 of the suspended membrane 13. The dielectric material 40 isdeposited by plasma enhanced chemical vapor deposition (PECVD). As shownin FIGS. 3D-3F, a substantial portion of the dielectric material 40 isdeposited using a wide angle of incidence (or arrival angle) so that thedielectric material is directed toward and builds up on the sidewalls ofthe openings. The angle of incidence at which the dielectric material isdirected toward the membrane 13 may be any width suitable to close theopenings 30 and seal the cavity 16. In one embodiment, the angle ofincidence is between about 30° and 60°, with 40° being preferred.

FIGS. 4A and 4B show a close up view of first and second structures 50 aand 50 b, respectively, and show the difference that the angles ofincidence may have during a deposition process. Each of the first andsecond structures 50 a and 50 b includes a material 52 formed on asubstrate 54, with the material 52 having an opening 56 and formingsidewalls 58. In FIG. 4A, a dielectric 60 is deposited using a wideangle of incidence, and in FIG. 4B, a dielectric 60 is deposited using asubstantially narrow angle of incidence that is nearly vertical. As canbe seen in FIG. 4A, when the angle of incidence is wide, the dielectric60 is directed toward and deposits onto the sidewalls 58 of the openings56. Conversely, as shown in FIG. 4B, when the angle of incidence isnarrow and more vertical, the dielectric 60 is not directed toward thesidewalls 58 of the opening 56 and thus does not build along thesidewalls 58. As shown in FIGS. 4A and 4B, a portion of the dielectric60 is deposited on the substrate 54, however, in FIG. 4A a smallerportion is deposited on the substrate 54 than in FIG. 4B.

The angle of incidence for deposition can be achieved by a number oftechniques. In one embodiment, the angle is set at one desired value,such as 40° from the vertical on one side, and the deposition carriedout for a selected time, such as 5 minutes. Then, the angle is changedto 40° from the vertical to the other side and deposition continues forthe same selected amount of time. The deposited layer 60 slowly buildsup as shown.

In an alternative embodiment, the angle varies continuously from +40° to−40° from the vertical for the entire deposition period.

FIGS. 3D-3F show various stages of the membrane MEMS device 20 duringthe PECVD process to close the openings 30. It is to be appreciated thatthese stages are not distinct stages of the process but rather areprovided herein for illustrative purposes to show how the openings ofthe membrane are closed. As shown in FIG. 3D, the dielectric material 40is directed toward the upper surface 26 of the membrane 13 with a wideangle of incidence. The dielectric material 40 adheres to the uppersurface 26 of the membrane 13 and has a cantilever portion that extendsbeyond the openings 30 thereby making the openings 30 smaller. Somedielectric material 40 goes through the openings 30 and deposits on theupper surface 22 of the substrate 14.

As shown in FIG. 3E, which is some time later, and in many cases secondslater, the dielectric material 40 continues to be directed toward themembrane 13 with a wide angle of incidence and is beginning to adhere tothe sidewalls 32 of the openings 30. The dielectric material 40continues to have a cantilever portion that extends beyond the openings30 making the openings smaller than shown in FIG. 3C. Some of thedielectric material 40 continues to be deposited on the upper surface 22of the substrate 14.

As shown in FIG. 3F, which may be seconds later, the dielectric material40 continues to be directed toward the membrane 13 with a wide angle ofincidence. More dielectric material 40 adheres to the sidewalls 32 ofthe openings 30 and to the upper surface 26 of the membrane 13 such thatthe openings 30 are closed, thereby sealing the cavity 16. The thicknessof dielectric material 40 is any thickness suitable to seal the openings30. In one embodiment, the thickness is 1.5 microns.

Again, some dielectric material 40 continues to be deposited on theupper surface 26 of the substrate 14. As can be seen from FIGS. 3D-3F,the width of the dielectric material 40 in the cavity 16 becomes smallerover time. This is due to the fact that the widths of the openings 30become smaller over time due to the cantilever of dielectric material 40extending into the openings 30.

It is to be appreciated that various dimensions of the membrane MEMSdevice may be adjusted to accommodate the dielectric deposition process.For instance, the height of the cavity may be an amount such that thedielectric deposited on the upper surface of the substrate does notprevent the membrane from functioning for its intended purpose. Forinstance, in the embodiment in which the membrane functions as apressure sensor, the height of the cavity should be of an amount thatthe when the membrane flexes inwardly toward the substrate due to apressure applied to the upper surface of the membrane, the lower surfaceof the membrane does not come in contact with the dielectric materialthat was deposited on the upper surface of the substrate.

The membrane dimensions may also be adjusted to accommodate thedeposition process. For instance, the higher the aspect ratio of theopening, i.e., the width of the opening versus the thickness of themembrane, the width the deposition angle of incidence must be. In thatregard, in many embodiments the aspect ratio of the openings is small.In one embodiment, the aspect ratio is less than or equal to 1. That is,the thickness of the membrane is equal to or greater than the diameter(or at least one dimension when not a circle) of the opening. In oneembodiment, the openings have a diameter that is less than 1 micron, andthe membrane thickness is equal to 1 micron. Similarly the spacingbetween the openings will typically depend on the size of the openings.That is, as the size of the opening goes up, the number of openings cango down. In one embodiment, the spacing between the openings is 1.5times the diameter of each opening. It is also to be appreciated thatthe thickness of the dielectric deposited will also depend on the sizeof the openings.

One benefit of sealing the cavity of the membrane MEMS device using aPECVD process is that in some embodiments, the pressure of the cavitymay be set while in the PECVD chamber. That is, the chamber in which thedeposition takes place may be pressurized at the same level in which thecavity is to be set. It is to be appreciated that various pressurelevels may be used, but as one example, the cavity of a pressure sensormembrane MEMS device preferably has a pressure of 3 to 4 Torr. In such acase, the chamber of the PECVD tool may pressurized to between about 3to 4 Torr during the deposition step.

In addition to the pressure setting advantage, various other benefitsmay be obtained by the present invention. One benefit discussed aboveincludes the ability to use a dry etch process to remove the sacrificiallayer and suspend the membrane. Using a dry etch process can preventvarious failures of the device, such as collapse of the membrane. Byusing a dry etch process, the dimensions of the openings may be reduced.Thus, by being able to limit the size of the openings, the ability toclose the openings by depositing the dielectric material along thesidewalls of the openings has increased. Furthermore, it should beappreciated that the methods described herein can be applied at a waferlevel and can be readily integrated into a current processing line.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. A method of forming a flexible membrane,comprising: forming a sacrificial layer over a surface of a substrate;forming a conductive electrode over the sacrificial layer, theconductive electrode including a plurality of openings with sidewalls;forming a cavity between the conductive electrode and the substrate byremoving the sacrificial layer through the plurality of openings in theconductive electrode; and closing the openings by depositing adielectric material on the sidewalls of the openings and on an uppersurface of the portion of the conductive electrode, and continuing todeposit the dielectric material for a time period after the openings areclosed to seal the cavity from atmosphere, wherein depositing thedielectric material includes alternatingly directing the dielectricmaterial between a first angle of incidence and a second angle ofincidence, the second angle of incidence being different from the firstangle of incidence, the deposited dielectric material and the conductiveelectrode together forming the flexible membrane.
 2. The method of claim1 wherein the first angle of incidence is between about positive 30° and60° and the second angle of incidence is between about negative 30° and60°.
 3. The method of claim 1 wherein depositing the dielectric materialcomprises depositing the dielectric material by chemical vapordeposition.
 4. The method of claim 1 wherein depositing the dielectricmaterial further includes depositing dielectric material on the outersurface of the released conductive electrode.
 5. The method of claim 1wherein depositing the dielectric material further includes providingportions of the dielectric material through the plurality of openingsand depositing the portions of the dielectric material on the surface ofthe substrate below the released portion of the structural layer.
 6. Themethod of claim 1 wherein forming the plurality of openings in thestructural layer having a width or a diameter equal to or less than athickness of the conductive electrode.
 7. The method of claim 1 whereinthe dielectric material has a thickness that is 1.5 times greater than athickness of the structural layer.
 8. A method of forming a flexiblemembrane, the method comprising: forming a conductive electrode over asurface of a substrate, the conductive electrode being spaced apart fromthe surface of the substrate and defining a cavity therebetween, theconductive electrode having a plurality of openings with sidewalls; anddepositing a dielectric material on the sidewalls of each opening and anouter surface of the conductive electrode to a sufficient depth to sealthe cavity thereby forming the flexible membrane, wherein depositing thedielectric material includes alternatingly directing the dielectricmaterial between a first angle of incidence and a second angle ofincidence, the second angle of incidence being different from the firstangle of incidence.
 9. The method of claim 8 wherein the pressure in thecavity is lower than atmosphere.
 10. The method of claim 8 whereinforming the conductive electrode over the surface of the substratecomprises removing a sacrificial layer through the plurality openings todefine the cavity.
 11. The method of claim 8 wherein each opening has awidth or diameter that is equal to or less than a thickness of theconductive electrode.
 12. The method of claim 8 wherein the cavity has adepth that is at least 5 times greater than a thickness of theconductive electrode.
 13. The method of claim 8 wherein the first angleof incident is positive and the second angle of incident is negative.